A gypsum board is known as a board which is constituted from a core mainly made from gypsum and sheets of paper for gypsum board liner covering the core. The gypsum boards are widely used in various kinds of buildings as architectural interior finish materials, because of their advantageous fire-resisting or fire-protecting ability, sound insulation performance, workability, cost performance and so on. In general, the gypsum boards are produced by a continuous slurry pouring and casting process. This process comprises a mixing step, a forming step and a drying and cutting step. In the mixing step, calcined gypsum, adhesive auxiliary agent, set accelerator, foam (or foaming agent), the other additives and so forth are mixed with admixtures and mixing water in a mixer. In the forming step, calcined gypsum slurry prepared in the mixer (referred to as “slurry” hereinafter) is fed into an area between upper and lower sheets of paper for gypsum board liner, so that a continuous plate-like and belt-like layered formation is formed. In the drying and cutting step, the continuous layered formation, which has dried and set to some extent on a conveyer device, is roughly severed and forcibly dried, and thereafter, cut to be a product size.
In such a process of producing the gypsum boards, the lower sheet of paper for gypsum board liner (referred to as the “lower sheet” hereinafter) is unwound from a paper roll of the sheet on a roll stand for the lower sheet. The lower sheet is continuously transferred by a forming belt (an upper belt track) of a belt conveyor device continuously moved. The slurry continuously discharged from the mixer is poured onto the lower sheet. A plurality of scores (creases or folding lines) are formed on right and left edge portions of the lower sheet by a scoring device, a grinding tool or the like, and then, the edge portions of the lower sheet are folded in line with the scores. On the other hand, the upper sheet of paper for gypsum board liner (referred to as the “upper sheet” hereinafter) is unwound from a paper roll of the sheet on a roll stand for the upper sheet, and the upper sheet is overlaid on the slurry. A gluing device, which applies or coats a quantity of glue to the right and left edge portions of the upper sheet immediately before the upper sheet is overlaid on the slurry, is provided on an apparatus for producing the gypsum boards. The gluing device includes a glue supply part which continuously applies or coats a predetermined quantity of glue to the edge portions of the upper sheet.
Each of the edge portions of the upper paper glued by the gluing device is aligned with each of the edge portions of the lower sheet and overlaid thereon, and the continuous three-layered formation comprising the upper and lower sheets and the slurry is fed to a forming device, such as forming plates or forming rollers. The continuous three-layered formation shaped in a form of plate-like continuous strip by the forming device is continuously transferred by the belt conveyer device, and it dries and sets to some extent thereon, and then, it is roughly severed, and thereafter, forcibly dried by a drying device for removing excessive water and cut to be the product size.
FIG. 12 includes partial cross-sectional views and a partial perspective view, each illustrating a structure of the edge portion of the aforementioned continuous layered formation W which has the upper and lower sheets 1, 2 glued. In FIG. 12(A), a cross-section of the edge portion suitably glued. In FIGS. 12(B) and 12(C), a configuration of the edge portion is shown in which a glue joint failure occurs.
When the upper and lower sheets are desirably joined together at a glue joint section G as shown in FIG. 12(A), an edge portion E with a rectangular cross-section is made, which encloses the slurry S in a state before drying and setting. The continuous layered formation W having a thickness t is conveyed in a conveyance direction of the forming belt, as shown by an arrow in FIG. 12(C). As illustrated as an exfoliation or gap K in FIGS. 12(B) and 12(C), the edge portion of the upper paper 2 may be partially separated from the edge portion of the lower paper 1, owing to an incomplete joint at the glue joint section G. Such an exfoliation K may occur immediately after gluing, or may occur during the drying and setting process of the slurry S. The exfoliation K slightly appearing immediately after gluing sometimes rejoins naturally, during the drying and setting process of the slurry S. Therefore, it is very difficult to locate the position where the exfoliation K occurs in the production line.
Further, such an exfoliation K tends to occur frequently in the production line for producing the gypsum boards with a high specific gravity equal to or greater than 0.9, compared to the production line for producing the gypsum boards with a standard specific gravity which is smaller than 0.9. Taking its cause into consideration, this is because a relatively thick paper with a large basis weight is used for the gypsum boards with high density and a weight or load of the slurry with high density acts on the lower and upper sheets.
Furthermore, also in a case of production of the light-weight gypsum boards having the specific gravity smaller than the specific gravity (the specific gravity in a range of 0.7 to 0.8) of the standard gypsum boards (referred to as the “light-weight gypsum boards” hereinafter), the paper with a heavy weight may be used for ensuring strength of the entire gypsum board. In a case where the light-weight gypsum boards are produced with use of such paper, the glue-joint failure may occur relatively frequently.
The gypsum board products with the exfoliations K remaining thereon have to be removed from the product line, as being irregular products or defective products which cannot be shipped. This results in increase of production loss and deterioration of production yield. For improvement of the yield of production in the gypsum board production process, it is desirable to surely detect the glue joint failure at an early stage, thereby eliminating or overcoming the cause of the glue-joint failure at the early stage, by means of adjustment or regulation of the gluing device.
In Japanese Patent Laid-Open Publication No. 2000-74646 (Patent Literature 1), a device for detecting an edge angle is disclosed, which detects an angle of an edge face (a side end face) with use of an optical detection means, in order to detect a defective shape-forming at the edge portion of the gypsum board. This device comprises a light source for laser light or the like, a CCD image capturing device, an image processing device, and so forth. The light source continuously irradiates the side edge zone of the aforementioned continuous layered formation with light for imaging, and the image capturing device receives the light reflected from the side edge zone and continuously captures the image of the edge portion. The image processing device carries out image processing of the image of the edge face to measure an apparent width of the edge, and detects the edge angle on the basis of the measured width of the edge.
In Japanese Patent Laid-Open Publication No. 5-346319 (Patent Literature 2), a surface inspection device is disclosed, which is intended to detect failures occurring on an edge portion or a surface of the aforementioned continuous layered formation, by means of optical detection means. This inspection device has a light projector and a light receiving device. The light projector emits linear or plane light toward the edge portion or the surface of the continuous layered formation for visually representing a bright line or pattern on the edge portion or the surface. The light receiving device receives the light reflected from the continuous layered formation to form an image of the bright line or pattern on the formation, and detects an inclination or variation of the bright line or pattern appearing on the edge portion or the surface by means of arithmetic processing or numerical analysis.